The present invention relates to an induction heating apparatus for use in home, restaurants, offices, factories, etc., and in particular, to an induction heating apparatus capable of heating an object to be heated made of aluminum or the like.
In an induction heating apparatus, an alternating current of a high frequency from 20 KHz to 60 KHz is made to flow through an induction heating coil (hereinafter simply referred to as heating coil) to generate a high frequency magnetic field. This high frequency magnetic field produces an eddy current in an object of heating, such as a pan, kettle or the like container placed in the vicinity of the heating coil, due to electromagnetic induction. The object to be heated is heated by Joule heat caused by the eddy current. The object to be heated is made preferably of a magnetic material, such as iron or stainless steel having magnetism, because electromagnetic induction is utilized in the induction heating. In recent years, induction heating apparatuses, such as induction heating cooking appliances, capable of heating an object to be heated of a pan, kettle or the like container (hereinafter simply referred to as pan) made of a non-magnetic material, such as aluminum or copper, have come into practical use and the scope of application of induction heating apparatuses has expanded.
In an induction heating apparatus, the direction of the eddy current generated in a pan during heating is opposite to the direction of the current that flows through the heating coil. Consequently, a repulsive force occurs between the pan and the heating coil due to magnetism. On the other hand, an electromagnetic force also works between the heating coil and the pan. In the case of the pan of a magnetic material such as iron, an attractive force due to magnetism occurs between the heating coil and the pan. In the pan made of a magnetic material, the attractive force is, in general, greater than the repulsive force, and therefore, the pan is attracted to the heating coil.
In the case of a pan made of a non-magnetic material (hereinafter referred to as a non-magnetic pan), such as aluminum or copper, however, only the repulsive force works without the above-mentioned attractive force. Therefore, in the case that the non-magnetic pan, with food therein, is so light in weight that the gravity thereof is smaller than the above-mentioned repulsive force. And a “pan floating phenomenon” occurs wherein the non-magnetic pan floats up and leaves from the heating coil due to repulsive force. When the pan floating phenomenon occurs, the pan may move on the top plate made of a heat-resistant glass plate or the like and provided above the heating coil for placing the pan. The non-magnetic pan is made of a material having a low magnetic permeability and a low electric resistivity, such as aluminum or copper. Therefore, it is necessary to make a high frequency current that is greater than in the case of an iron pan to flow through the heating coil in order to secure approximately the same amount of heat as in the case of an iron pan or the like. Consequently, the above-mentioned repulsive force becomes greater than in the case of an iron pan, and the pan floating phenomenon easily occurs.
A first prior art concerning the usage of a pan of a non-magnetic material in an induction heating cooking appliance is shown in Japanese unexamined patent publication S61 (1986)-128492. According to this first prior art, a weight sensor for detecting the weight of a pan is provided in the surface of the top plate so that the weight of the pan is detected. In addition, the high frequency current flowing through the heating coil is detected by a current transformer so that the material of the pan is detected based on this detection output. In the case that the weight of the pan, including its contents, is a predetermined value or less and the material of the pan is aluminum or copper, since the pan floating phenomenon is liable to occur, the high frequency current in the heating coil is cut away so that heating is stopped. When the weight of the pan exceeds a predetermined value and, even in the case that the material of the pan is aluminum or copper, since there is no risk of the occurrence of the pan floating phenomenon, a high frequency current is made to flow through the heating coil so that heating is carried out.
A second prior art concerning the usage of a pan of a non-magnetic material in a induction heating cooking appliance is shown in Japanese unexamined patent publication S62 (1987)-276787. According to this second prior art, the weight of the pan and the material of the pan are detected in the same manner as in the above-mentioned first prior art. In the case that the material of the pan is a non-magnetic material having a high conductivity, for instance aluminum, the frequency of the high frequency current is raised to 50 KHz (20 KHz in the case of an iron pan) so that approximately the same amount of heat as in the case of an iron pan can be gained even in the case that such a pan is used. In addition, the high frequency current flowing through the heating coil is adjusted in accordance with the weight of the pan so as to limit to a current value wherein a range in which the pan floating phenomenon does not occur.
When cooking is carried out using a pan, temperature control is carried out so that the temperature of the object to be heated in the pan is maintained at the desired value during cooking, and consequently an appropriately cooked object is obtainable without risk of burning food material (object to be cooked). In particular, in the case that cooking oil is placed in a pan in order to cook tempura (Japanese fried food), it is important to maintain the temperature of the cooking oil at an appropriate level in order to cook good taste tempura. Though temperature control during cooking is not shown in the above-mentioned first and second prior arts, an induction heating cooking appliance having such a temperature control function is in practical use.
In the induction heating cooking appliance of the above-mentioned first prior art, the high frequency current is cut when the weight of the pan including its contents is a predetermined value or less, and therefore, cooking using a non-magnetic pan cannot be carried out. A non-magnetic pan, for example, cannot be used for cooking a small amount of food material for a family of a small number of people.
In the induction heating cooking appliance of the second prior art, the current flowing through the heating coil is restricted in relation with the weight of the pan with its contents inclusive. Therefore, the user cannot cook with a light pan for a small amount of food material.
It is necessary to detect the temperature of the food material in order to control the temperature of the food material in the pan. It is not easy, however, to directly measure the temperature of the food material. Therefore, the temperature of the bottom surface of the pan is, usually indirectly measured by means of a temperature sensor and the temperature of the food material is indirectly measured. The temperature sensor is provided on the lower surface of a top plate whereon the pan is placed. The temperature of the bottom of the pan is detected by the sensor through the top plate when the pan is placed on the top plate. Correct temperature detection is carried out only when the pan makes contact with the top plate. In the case that a non-magnetic pan is used, the pan floating phenomenon may lead to a state that the pan floats and moves on the top plate shifting away from the correct position. Then, the temperature sensor cannot correctly detect the temperature of the bottom of the pan. Under such state, the temperature sensor provides a detection output indicating an erroneous low temperature to the control section, and thus the control section increases the high frequency current supplied to the heating coil, to increase the temperature. Such being the case, correct temperature control is not carried out, and there is a risk wherein the temperature of the pan and the temperature of the object to be heated may rise in an abnormal level.
A general example of a conventional induction heating cooking appliance is described with reference to FIG. 8.
In FIG. 8, a heating coil 53 is disposed below a top plate 52 which has an object 51 thereon, such as a pan to be heated. A high frequency current is supplied from an inverter circuit 55 as a heating coil output adjustment section to a heating coil 53, and the heating coil 53 carries out induction heating by applying the magnetic field caused by a high frequency alternating current to the object 51 to be heated. A temperature sensor 57 is provided at approximately the center portion of heating coil 53 so as to make contact with the lower surface of top plate 52 and detects the temperature of the center portion of object 51 to be heated via top plate 52. Detected temperature of object 51 to be heated, which has been detected by temperature sensor 57, is sent to a temperature control section 58. Temperature control section 58 controls the operation of the inverter circuit 55 so that the temperature detection value becomes equal to the control target temperature (see, for example, Japanese unexamined patent publication H7 (1995)-254483 (FIG. 1, pages 4 to 6)).
The operation of the conventional induction heating cooking appliance described above is herein described in detail.
The inverter circuit 55 as the heating coil output adjustment section rectifies, smoothes and converts the alternating current of a commercial frequency inputted from a conventional power supply (not shown) to direct current, and then converts the direct current to a high frequency current of the desired frequency. The high frequency current is supplied to the heating coil 53. The heating coil 53 generates an eddy current in the object 51 to be heated, for example, a pan magnetically coupled with heating coil 53, so that object 51 to be heated is heated with Joule heat.
When object 51 to be heated is induction heated, temperature sensor 57 detects the temperature of object 51 to be heated and temperature control section 58 controls the output of the inverter circuit 55 so that the temperature of object 51 to be heated becomes equal to the control target temperature.
In the conventional induction heating cooking appliance, heating coil 53 is generally in a spiral form, wherein the inner diameter thereof is approximately 50 mm and the outer diameter is approximately 150 mm. Heating coil 53 is placed at a distance of approximately 3 mm away below from top plate 52.
In the above-mentioned conventional induction heating apparatus, temperature control section 58 changes value of the high frequency current flowing through heating coil 53. This current is the output of inverter circuit 55, in accordance with the temperature detected by temperature sensor 57, and controls the heat output of object 51 to be heated. At this time, heating coil 53 itself emits heat, and therefore, the temperature of heating coil 53 changes in accordance with the high frequency current value flowing therethrough. Accordingly, the temperature detected by temperature detector 57 which is provided in the vicinity of heating coil 53 is affected by the temperature of object 51 to be heated, and the temperature of heating coil 53 itself. Therefore, temperature sensor 57 cannot precisely detect the temperature of object 51 to be heated, and temperature control suffers harmful influences. This is a significant problem, wherein such a structure is disadvantageous in order to achieve successful cooking when using the induction heating cooking appliance.